304 APPLIED SCIENCE 



Steel instead of iron is used in the construction of modern 

 boilers. Although the average diameter of boilers has in- 

 creased only slightly and the average length has even de- 

 creased, the modern type is capable of carrying three times 

 as high a pressure as the old type. The diameters of modern 

 boilers range from 48 to 69 in., and the lengths from 16 to 

 20ft., but a boiler carrying a pressure as high as 150 Ibs. 

 per square inch is not at all uncommon; some carry even a 

 much higher pressure. 



345. Water-Tube Boiler. The water-tube boiler is the 

 result of a demand for high pressures of steam. In this type 



contained in tubes 

 which, on account of 

 their comparatively 

 small size, reduce the 

 thickness of metal, 

 the quantity of water 

 contained, and con- 

 sequently the total 

 weight of -the boilers. 

 At the same time the 

 small tubes increase 

 the rapidity with 

 which steam can be 

 generated without in- 



FIG. 159. Marine Boiler. The tubes are jury from unequal 

 of small diameter and shorter than in . TTT , 



land type. Oil may be burned in this expansion. Water- 

 boiler. In this boiler the entire surface tube boilers are in 

 is composed of fire-brick. 



extensive use for both 



stationary and marine work (Fig. 159). They are more com- 



